This invention relates to plasma etchers which are used to make integrated circuits; and more particularly, it relates to fixtures for use in cleaning such etchers.
A cross section of a prior art plasma etcher 10 which the present invention can clean is shown in FIG. 1. This plasma etcher 10 includes a holding member 11, an enclosing member 12, gas ducts 13, a voltage generator 14, a capacitor 15, and an insulator 16 which electrically separates members 11 and 12. Typically, member 12 is bell jar shaped and is about three feet high and three feet in diameter. Also typically, member 11 is hexode shaped and is less than half the size of member 12.
In operation, the enclosing member 12 is opened (such as by lifting it off its base 12a), and integrated circuit wafers 17 which are to be etched are placed on the holding member 11. Thereafter the enclosing member 12 is closed and various gases which form a plasma 18 are introduced via the ducts 13 into the space which member 12 encloses. Generator 14 then generates a voltage at a radio frequency (such as 13.55 MHz) which together with capacitor 15 causes a DC bias voltage to be built up in the plasma 18.
Curve 20 in FIG. 2 shows how the bias voltage in plasma 18 varies between the enclosing member 12 and holding member 11. That bias voltage starts at 0 volts on the surface of the enclosing member 12; then it quickly increases to about 30 volts as the plasma is entered; then it stays constant until the surface of holding member 11 is nearly reached; and then it quickly drops to about -500 volts on the surface of member 11. Such a large voltage change near holding member 11 is very desirable because it accelerates the ions in plasma 18 towards the wafers 17 with a high velocity to thereby produce the desired wafer etch.
By comparison, only a small change occurs in the voltage bias near the enclosing member 12. Consequently, the plasma ions are only accelerated slowly towards the enclosing member, and that enables the plasma ions to attach themselves to the enclosing member and gradually build up as a polymer film 19. This film 19 must be periodically cleaned off of member 12 or it will adversely affect the etching of the wafers. For example, the presence of film 19 will decrease the etch rate of the wafers, and particles of film 19 can fall off into the plasma and contaminate the wafers.
In the prior art, the film 19 was removed by a workman who would open the enclosing member 12 and physically scrub the film off. However, such a removal process can be very hazardous because the chemicals which form plasma 18 and the film 19 usually are toxic. For example, plasmas containing methane and chlorine are used to etch metal; and plasmas containing methane and fluorine are used to etch SiO.sub.2. Consequently, the workman who scrubs off the film 19 must wear a gas mask and protective clothing.
Accordingly, a primary object of the invention is to provide a fixture for cleaning plasma etchers in a less hazardous fashion.